There is hitherto known technology called the adaptive cruise control (ACC) which detects a following distance between an own vehicle and a preceding vehicle with a ranging sensor such as a laser radar, and which controls automatically the following distance to the preceding vehicle, thereby performing the following traveling.
In recent years, there is known technology as disclosed by Patent Literature 1 which performs preceding vehicle following traveling using a compensated approaching/separating state evaluation index KdB_c, obtained by compensating an approaching/separating state evaluation index KdB in consideration of the velocity of a preceding vehicle. The approaching/separating state evaluation index KdB is an index which expresses the change degree per unit time of the area of an assumed image of an object in front, to be viewed by eyes of the driver.
The approaching/separating state evaluation index KdB and the compensated approaching/separating state evaluation index KdB_c described above are expressed, for example, by Equations 1 and 2 in the following, respectively. In Equations 1 and 2, D is a following distance between an own vehicle and a preceding vehicle, Vr is a relative velocity of the preceding vehicle to the own vehicle, a is a multiplier, and Vp is a velocity of the preceding vehicle.
                    (                  Equation          ⁢                                          ⁢          1                )                                                            KdB        =                  10          ×                                    log              10                        ⁡                          (                                                                4                  ×                                      10                    7                                    ×                                      Vr                                          D                      3                                                                                                  )                                                          (                  Eq          .                                          ⁢          1                )                                (                  Equation          ⁢                                          ⁢          2                )                                                                      KdB_c          ⁢                      (            a            )                          =                  10          ×                                    log              10                        ⁡                          (                                                                4                  ×                                      10                    7                                    ×                                                            Vr                      -                      aVp                                                              D                      3                                                                                                  )                                                          (                  Eq          .                                          ⁢          2                )            
As shown in Equation 1, the compensated approaching/separating state evaluation index KdB_c becomes greater as the absolute value of the relative velocity Vr approaching the preceding vehicle becomes greater. It becomes smaller as the velocity of the preceding vehicle Vp is higher and becomes greater as the following distance is shorter. Since the following distance D appears in a cubic term, the increasing gradient of the compensated approaching/separating state evaluation index KdB_c to the decreasing change of the following distance D becomes steeper as the following distance D becomes shorter. As disclosed by Patent Literature 1, when the preceding vehicle following traveling is performed by use of the compensated approaching/separating state evaluation index, it is possible to perform the following traveling with little feeling of discomfort.
However, when following a preceding vehicle by the ACC and entering a curve way, there arises the following issue. Specifically, when the preceding vehicle travels in a curve way at a high speed with excessive lateral acceleration, the own vehicle enters the curve way without decelerating sufficiently so as to maintain the distance to the preceding vehicle. Accordingly, the own vehicle is also forced to travel in the curve way at a high speed, following the preceding vehicle. Therefore, there is an issue that a driver will feel fear or the ACC will be canceled by the driver stepping on a brake because of the fear.
As other issues, in the ACC, when the preceding vehicle runs out of a detection range of the ranging sensor of the own vehicle, the own vehicle enters automatically into traveling at a set vehicle velocity (that is a vehicle velocity set by the driver). Accordingly, when the preceding vehicle runs out of the detection range of the ranging sensor of the own vehicle in the state where the own vehicle is traveling at a speed less than the set vehicle velocity of the own vehicle until just before entering a curve way, the own vehicle will be accelerated up to the set vehicle velocity of the own vehicle, and sufficient deceleration may not be attained at the time of entering the curve way. Accordingly, the own vehicle is forced to travel in the curve way at a high speed, and therefore, there is an issue that a driver will feel fear or the ACC will be canceled by the driver stepping on a brake because of the fear.
As a means to solve the present issue, Patent Literature 2 discloses technology which enables deceleration control corresponding to the feeling of danger of the driver at the time of entering a curve way, by use of the approaching/separating state evaluation index KdB. When explained in detail, in the technology disclosed by Patent Literature 2, the distance between an own vehicle and the road boundary of a curve way existing ahead of the own vehicle or a road collateral object provided in the neighborhood is acquired by use of a ranging sensor such as a laser radar. Next, in consideration of the acquired distance to the road boundary or the road collateral object and the velocity of the own vehicle, the compensated approaching/separating state evaluation index KdB_c is calculated to be used as the starting timing of deceleration corresponding to the feeling of danger of the driver at the time of entering the curve way. Then, by determining the deceleration starting timing before entering the curve way by use of the calculated compensated approaching/separating state evaluation index KdB_c, it is possible to start the deceleration control to the timing corresponding to the feeling of danger of the driver at the time of entering the curve way.
However, when the distance to the road boundary of the curve way existing ahead of the own vehicle or the road collateral object provided in the neighborhood cannot be acquired by an autonomous sensor such as a ranging sensor, the technology disclosed by Patent Literature 2 raises an issue that it is difficult to start the deceleration control to the timing corresponding to the feeling of danger of the driver at the time of entering a curve way. For example, when employing a ranging sensor such as a laser radar as the distance acquisition means, there may be occasions when it is difficult to acquire the distance to the road boundary or the road collateral object due to interruption by the preceding vehicle.
In the technology disclosed by Patent Literature 2, the deceleration control is started to the timing corresponding to the feeling of danger of the driver at the time of entering a curve way; accordingly, when the preceding vehicle enters the curve way at a speed slower than needed, the following distance to the preceding vehicle concerned may become too short, and it is likely to induce feeling of discomfort to the driver.    (Patent Literature 1) Japanese Patent Laid-open No. 2008-280017    (Patent Literature 2) Japanese Patent No. 4289421